Integrated circuits, comprised of numerous circuit elements, are typically fabricated in layers on the surface of a semiconductor wafer. Many fabrication processes are repeated numerous times, constructing layer after layer until fabrication is complete. Metal layers (which typically increase in number as device complexity increases) include patterns of conductive material that are insulated from one another vertically by alternating layers of insulating material. Vertical, conductive tunnels called “vias” typically pass through insulating layers to form conductive pathways between adjacent conductive patterns.
Periodically, an electrical malfunction or design flaw is found when an integrated circuit is electrically tested. Implementing a design change can be an extensive process. Typically, among other tasks, a circuit designer may have to produce new schematics, a vendor may need to supply new masks or other fabrication supplies, and wafer fab personnel may need to implement new process flows on various equipment sets. Rather than commencing a lengthy and costly redesign process only to have the new design fail in operation, it is often preferable to modify and test a physical sample of the integrated circuit prior to formalizing the modified design.
Integrated circuit failure analysis often involves the use of several different types of equipment, or tools. One of the most versatile failure analysis tools is the focused ion beam (FIB) apparatus, which can facilitate device modification. The FIB is a tool including one or more ion columns for generating ion beams. In general, the FIB is used for performing integrated circuit repair, editing, cross-sectioning, modifications to aid microprobing of the integrated circuit, and other common failure analysis applications. As an aside, it is noted that a device may need to be preprocessed before being operated on by the FIB tool. For example, a packaged device may need to be decapsulated, or “decapped,” and an etching or grinding process for removing the encapsulant above the die may need to be performed prior to operations by the FIB tool.
Referring now to FIG. 1, a side view is shown of an FIB apparatus 100 in operation. The centerpiece of a conventional FIB 100 is the primary ion column 102, which generates an ion beam 104 from a liquid metal ion source—typically gallium. Positively charged gallium ions (Ga+) 106 are drawn off a field-emitter point source and accelerated by the application of a large potential, generally in the 30-50 kilovolt (kV) range. With the aid of electrostatic lenses, the emission is focused into a beam 104 typically having a sub-micron diameter. The ion beam 104 can be used to mill through a sample integrated circuit 108, as may be required in failure analysis. The sample 108 is usually positioned on a stage 126 inside a vacuum chamber 128.
Typically, secondary electrons 110, secondary ions (i+ or i−) 112, and neutral molecules and atoms 114 are ejected from the sample surface 116 when the ion beam 104 impacts the sample 108. The charged particles are drawn toward an electrically-biased grid and collected by a detector (not shown) generally positioned at an angle from the column 102. The signal from the ejected particles may be amplified and displayed to provide a real-time image of the area of interest.
Dual-column tools may have an ion column 102 complemented by an optional electron column 120, which is typically inclined 45-60 degrees from the ion column 102. The electron column 120 delivers a flood of electrons 122 to the local area and performs scanning electron microscope (SEM) imaging for the tool 100, providing an image generally superior to that formed by the ion column 102 alone. The electron column 120 may also aid in cross sectioning and transmission electron microscope (TEM) sample preparation, due to the ease of imaging the milling area.
The ion beam 104 is generally moved across the sample 108 in a single-direction raster or in a user-defined pattern. The operator has control over various parameters, such as beam current, spot size, pixel spacing, and dwell time. The dose, or amount of ions 106 striking the sample surface, is generally a function of the beam current, duration of scan, and the area scanned. The secondary yield, which is the number of ejected ions 112 per primary ion 106 directed at the sample, is a function of the material being milled. The amount of surface material of the sample 108 sputtered away by the ion beam 104 is a function of all the above-mentioned parameters.
While the ion beam 104 itself typically has a sputtering effect on the sample materials, there is often a need to add gases to assist in chemically removing material, thereby enhancing material removal process. Gas-assisted etching is a common feature in modern FIBs. An optional gas injection column 130 delivers a localized gas 132 to the area to be milled. This deposition gas 132 can interact with the primary ion beam 104 to provide selective gas-assisted chemical etching. Alternatively, the primary ion beam can be used to decompose the gas to provide selective deposition of conductive or insulating materials on the sample.
Semiconductor device modification can be facilitated by the FIB by directing the ion beam at a localized area of the modification to be performed. The ion beam removes material in the local area, milling through the various layers. When the layer of interest is reached, circuit edits can be performed by depositing a new metal line or other material in a desired location to establish a connection, or by cutting through an existing conductive line to sever a connection.
As a typical integrated circuit consists of alternating layers of conducting material and insulating dielectrics, with many layers containing patterned areas of both, the milling rate and effects of ion beam milling vary vastly across the device. In most device modification operations and several other FIB functions, it is preferable to stop the milling process as soon as a particular layer is exposed. Imprecise endpointing can greatly reduce the chances of success for a given device edit operation due to the potential of inadvertently creating either opens or shorts in the circuits.
Consequently, precise endpoint detection for the milling operation is desired. Determining an endpoint for the milling operation, or the instance at which the beam has reached the layer of interest, is becoming more difficult as devices grow in complexity and are designed with a greater number of layers. Endpoint detection is one of the most difficult tasks with which a failure analyst is faced.
Various methods exist for detecting FIB etch endpoints. Presently, the most common method is observation of the secondary electron yield, or the electrons ejected. The secondary yield is usually manifested as a contrast in the image formed from the secondary electrons (higher electron counts per unit time typically yield greater contrasts). As there is a significant difference in the electron yield between conductors, such as copper and aluminum relative to dielectrics, the endpoint of a certain milling/etching operation can be determined from observing the resulting live image. An offshoot technique is monitoring the secondary ion yield, or the ions ejected. The aforementioned methods may be suitable for surface milling, but on advanced integrated circuits with several metal layers, the relatively high aspect ratios of the complex edits can make it very difficult to obtain sufficient image contrast for precise endpoint detection.
On some FIBs, end-point detection is performed through the addition of an Auger, SIMS (Secondary Ion Mass Spectroscopy), or EDX (Energy Dispersive X-ray) detector to monitor material changes. This practice generally necessitates the use of another port on the FIB for each of these detectors. As ports are generally limited on FIBs and may be needed for other operations, port availability can often be an issue. End-point determination can also be performed by monitoring the substrate current as a function of the milling process. When milling silicon wafers with an FIB, the wafer backside is generally grounded to a stage within the apparatus, so measuring the current passing through the stage can generally give an accurate representation of the substrate current. When the ion beam impacts the surface of the device, a charge build-up occurs when dielectric materials are struck. When the milling reaches a conductive material, the charge is largely dissipated through the substrate. By monitoring the current through the substrate, one can determine if a conductive material has been reached. Alternatively, with packaged devices, the leads on the chip may be shorted to the stage and the total lead current is thus monitored.
Yet another end point determination method involves using a voltage contrast feature of FIBs to identify active conductors. Applying a voltage to the conductive layer of interest greatly impacts the secondary yield, and thereby changes the image contrast. The secondary emission characteristics for a conductor tied to ground vary significantly when compared to a conductor at a different potential. This variation produces a passive voltage contrast in the image. Active voltage contrast can be obtained by toggling the voltage on the conductor of interest. When modifying a packaged device, the voltages may be applied via an external package lead that is interconnected with the layer of interest. However, provide a wired connection can be a difficult task to achieve when modifying a wafer, i.e., a device that is not packaged. Applying a voltage directly to the layer of interest within a wafer is much more complicated.
Recently, there have been inventions and papers discussing the automation of the endpointing technique to stop the edit process at the desired level. Despite recent advancements, endpoint detection is still one of the most difficult steps in device modification and is often the reason for failures in the FIB edit and modification process. As devices continue to increase in complexity and number of layers, successful endpoint detection will become even more difficult to achieve. A more precise method of endpoint detection is therefore desired.